1. Field of the invention
The present invention relates to a ramp generator. Such a ramp generator is preferably for generating a xe2x80x98solidxe2x80x99 used in a video wipe generator of a vision mixer.
A solid is an electrical signal representing a three dimensional surface of a desired shape. It comprises at least one ramp signal and typically comprises a combination of at least two ramp signals which themselves may be modified. It may also comprise a signal defined by a polar coordinate system and which represents a curved surface.
2. Description of the Prior Art
Reference will now be made to FIGS. 1 to 3 of the accompanying drawings which show background to the present invention.
FIG. 1 illustrates a known simple wipe between two video sources X and Y.
As the wipe proceeds as indicated by arrow W, video X is replaced across the display by video Y (or vice versa). The effect of a wipe is achieved by mixing the video sources X and Y according to
KX+(1xe2x88x92K)Y
where K is a keying signal. The keying signal K is derived from a xe2x80x98solidxe2x80x99. This will be explained with reference to FIGS. 2 and 3 i.e. a function having a value depending on the h and v co-ordinates within the picture, where v represents line number and h represents pixel position along a line.
FIG. 2 a illustrates a known example of a xe2x80x98solidxe2x80x99 which is a simple ramp. As shown in FIG. 2, a clip level CP is defined. It will be appreciated that over a field or frame, the clip level defines a plane referred to herein as the clip plane, which will be described in more detail with reference to FIG. 4 below. The keying signal K is, in known manner, derived from the solid by applying high gain to the solid and limiting the result, as shown in FIG. 2B. The keying signal has two levels 0 and 1. The transition between the levels occurs where the solid intersects the clip plane CP. The position of intersection is varied, to produce the wipe, by adding an offset to the solid.
FIG. 3 is a schematic block diagram of a wipe generator of a vision mixer comprising a solid generator, a clip element, a gain element, a limiter and a mixer which mixes video sources X and Y in dependence upon the keying signal K.
The solid generator produces a solid, for example a ramp as shown in FIG. 2A. The clip element applies an offset to the ramp to vary the intersection of the ramp with he clip plane CP as shown in FIGS. 2A to 2C. Gain is applied to the offset ramp, in he gain element and the result limited in the limiter to produce the signal K. The mount of gain applied may be varied as shown in FIG. 2B: that varies the slope of the transition between the limit values of the keying signal K.
The mixer mixes the video sources X and Y according to
KX+(1xe2x88x92K)Y.
This is if K=1, the output is X, if K=0 the output is Y.
If the gain applied to the solid is unity and the clip offset is zero, the solid and the keying signal are identical.
The present invention concerns the solid generator.
The example of FIGS. 1 and 2 for ease of explanation produces a solid, a ramp, which varies as a function of only pixel position h along a line. It is necessary to produce solids which vary as a function of both h and v co-ordinates in a picture where v is line number. Previously this has been done using two parallel connected linear ramp generators one producing a ramp varying only with h and the other only v. However this arrangement has proved to be relatively inflexible.
According to the present invention, there is provided a video ramp generator which produces a video ramp signal R for each pixel of a predetermined set of pixels h where h=0 to n of each of a predetermined set of lines v where v=0 to m, wherein
R=Ah+Bv+C
where A, B, and C are coefficients which are signed values.
A preferred embodiment of the generator, comprises
means for storing the coefficients A, B, and C and first and second accumulated values,
adding means for incrementing the first accumulated value by A and the second accumulated value by B, and
control means by which, for each of the said lines v, the second accumulated value is incremented by the adding means by B to form C+Bv which is stored as both the first and the second accumulated values, and for each pixel of the said set of pixels on each of the said lines the first accumulated value is incremented by A by the adding means to form
C+Bv+Ah
xe2x80x83which is stored as the first accumulated value and is output as R.
Such a ramp generator provides great flexibility. The storing means, which may be registers, allow the ramp coefficients A, B and C to be easily changed. The change, in illustrative embodiments, takes place once per frame.
Changing the coefficients allows the ramps to be varied. For example it may be rotated by changing A and B. The ramp can be offset by changing C. The ramp generator avoids the use of multipliers.